Natural materials such as timber have conventionally been used as main construction materials, but wooden synthetic construction materials that are molded from a mixture of a wood powder and a thermoplastic synthetic resin have recently become known and been used as exterior construction materials and interior construction materials. Such wooden synthetic construction materials have attracted much attention because they enable the effective use of wastes (wastes of natural wood or synthetic construction materials) and make it possible to save oil resources serving as source materials and reduce the production cost.